Most of the conventional reclining type dental chairs in general use are such that the back seat thereof is selectively adjustable at limited angles, with its bent parts positioned around the underneath hip portion of the patient, and his knee joint, as is clearly illustrated in FIG. 1 of the accompanying drawings. Thus when a patient sits on the dental chair, his hip is sunken deep in the lowermost recess of the bent part, his oral cavity being not always oriented in a proper position subject to his stature and height of his upper half body. This has a result that the patient must move himself sliding along the seat of the dental chair either up or down to a position P as shown in FIG. 4 of the accompanying drawings thereby to orient his oral cavity in the position most suitable for dental treatment. Presuming a child sits on the aforementioned dental chair his head is placed on the back seat thereof to hinder the dental treatment so that a dentist is unable to freely undertake the treatment, being forced to assume an unnatural posture bending himself downward or peeping into the oral cavity of the child from one side only. Such being the case, the dentist cannot exactly manipulate treating instruments, often failing to treat the child in the most effective manner.
In the meanwhile, such tube instruments for example as a syringe or a vacuum pump are provided to be drawn out, and pulled into, the head rest of a dental chair through both sides thereof. However, the back seat of the dental chair is not held always at a fixed angle of inclination, the tube instruments must be drawn out or pulled into said sides from different positions for each use. This has still the result that a dentist cannot treat a patient in his best way by using the instruments.